Effect of partial orientation in [100] direction on the magnetic properties of Co-ferrite prepared from nano particles

American Journal of Applied Sciences, Jan, 2009 by H.M. El-Sayed

INTRODUCTION

Co-ferrites have attracted much attention in recent years as one of the candidates for high density magnetic recording and magneto-optical recording media because of their unique physical properties such as high Curie temperature, large magnetic anisotropy, moderate magnetization, excellent chemical stability and large Kerr and Faraday rotations (1), (2). However, the important problems, that the researchers are concerned with, are how to make Co-ferrite which has high coercivity, perpendicular anisotropy and small grain size for high density magnetic recording media.

Many ways are used for improving the magnetic properties of Co-ferrite. One of these methods is the divalent ions substitution e.g., Ni, Zn and Cd on the account of Co ions (3), (4), (5). But in fact this way gave a limited enhancement in the magnetic properties of Co-ferrite because the produced particles have large size and the formation of super-paramagnetic clusters.

One of the promising methods to improve the magnetic properties of Co-ferrite is the preparing of the material from its nano particles where, as the particle size decreases the magnetic coercivity increases (6).

The objective of the present research is to improve the magnetic properties of Co-ferrite by making an induced magnetic anisotropy during the preparation of the samples from its nano particles. This will be done by applying an external magnetic field during pressing the powder of magnetic nano particles of Co-ferrite before the final sintering.

PREPARATION OF THE SAMPLES AND EXPERIMENTAL TECHNIQUES

Co-ferrite nano particles powder was prepared by Chemical co-precipitation method. A pure chemical reagents of [Fe.sub.2][([SO.sub.4]).sub.3]. 5[H.sub.2]O and CoS[O.sub.4] were first dissolved in bi-distilled water and the Co/Fe molar ratio was fixed to 1/2. An alkaline solution (NaOH) was added to the salt solution until the pH was adjusted to 12.5. The solution was heated at 90[degrees]C with continuous stirring for 1 h. The co-precipitated powder was filtered and washed many times with bi-distillated water and dried in an oven in air atmosphere at 120[degrees]C.

For preparing the bulk sample, the powder was pressed, in tablet (for x-ray and porosity measurements) and cubic ( for magnetization measurements) shapes, at 3x[10.sup.8] Pascal. The samples were pressed in the presence of external magnetic field ([H.sub.ext]) which was parallel to the pressing direction. Four samples were prepared at different values of the external field ([H.sub.ext] = 0.0, 12KA/m, 16 KA/m and 20 KA/m). The final sintering temperature was 1000[degrees]C for 6 h.

X-ray diffraction patterns were performed using a diffractometer of type X-Pert Graphics and identified with Cu-[K.sub.[alpha]] radiation. The theoretical x-ray density ([d.sub.x]) of the samples was calculated using the formula ([d.sub.x] = 8M/N[a.sup.3]) where, M is the molecular weight, N is Avogadro's number and a is the lattice parameter. The density (d) of each sample was measured in bi-distilled water using Archimedes principle. The porosity percentage P (%) was calculated according to the relation

P = 100(1-d/[d.sub.x])%.

The magnetization (M), at room temperature and the hysteresis parameters were measured using the vibrating sample technique. The magnetizing field ranged from 0.0 up to 12 K Oe.

The particle size ([D.sub.hkl]) of the powder sample was measured from the x-ray chart according to Debye-Sherrer formula [7] which is given by,

[D.sub.hkl] = 0.9[lambda]/[beta]cos[theta]

where, [lambda], is the wave length of the used x-ray ([lambda] = 0.154 nm), [beta] is the half width and [theta] is the half diffraction angle. The particle size was determined by taking the average of the strongest peaks [D.sub.220], [D.sub.311], [D.sub.400], [D.sub.511] and [D.sub.440]. Also, the Scanning electron microscope (SEM) was used for measuring the grain size of the investigated samples.

RESULTS AND DISCUSSION

X-ray analysis: X-ray diffraction of the powder sample is shown in Fig. 1a. It is obvious that, the prepared sample from the chemical co-precipitation method has one cubic spinel phase. The lattice parameter was found to be about 8.383 A[degrees] which is in good agreement with that reported by Armulmurugan et al. and Pandya et al.,(8), (9). Also, from x-ray pattern, the average particle size of the powder sample is found to be about 18 nm. This result confirms that, the prepared sample has nano particle size. Figure 1b shows the image of SEM of the powder sample. It is noticed that, the powder sample has nano size particles in the range of 12 nm which is in a good agreement with x-ray measurements.

[FIGURE 1 OMITTED]

The x-ray diffraction patters of the bulk samples are shown in Fig. 2. It is clear that, all patterns have single spinel phase. It was found that, the lattice parameter of all samples is in the same order of the powder sample. Furthermore, it is valuable to note that, the relative intensities of (100) and (400) planes increase with increasing the applied magnetic field during pressing the samples ([H.sub.ext) while, at the same time, the relative intensities of the (220) and (440) decrease. This may be discussed as follows: